Metallic nano-structures have been demonstrated with unique abilities to manipulate light at the nano-scale. Owing to the coupling of the light with surface plasmons (SP) at the interface between a metal and a dielectric, metallic sub-wavelength structures can display strong color properties which are widely tunable by changing the structure geometries. Ebbesen et al. (U.S. Pat. No. 6,040,936, Mar. 21, 2000) teaches that a metal film having periodic arrays of sub-wavelength apertures shows extraordinary optical transmission and can serve as color filters. Light transmitted through the modulated filters can be combined to display any visual color. Kaminska et al. (U.S. Pat. No. 8,253,536B2, Aug. 28, 2012), having a common co-inventor as the present application, teaches that a metal film having sub-wavelength apertures can be patterned to display a color image which can be applied towards security applications. Kumar et al. (Nat. Nanotechnol. 7, 557, 2012) teaches that sub-wavelength structures comprising apertures and disks can serve as bright color pixels that allow for printing a color image in a resolution comparable with optical diffraction limit. Moreover, Wu et al. (Sci. Rep. 3, 1194, 2013) teaches that nano-cavities in a metal film can display angle-insensitive primary colors.
Nano-grating structures have been used in displaying optical information. Fattal et al. (Nature 495, 348, 2012) teaches that 1-D nano-gratings can be patterned to function as a multi-directional backlight to display multi-view color images. Nano-gratings have been used in authentication and anti-counterfeit applications, such as those demonstrated by Lai et al. (U.S. Pat. No. 7,113,690 B2, Sep. 26, 2006) and Schnieper et al. (U.S. Pat. No. 7,787,182 B2, Aug. 31, 2010; U.S. Pat. No. 8,270,050 B2, Sep. 18, 2012).
Production of color images using sub-wavelength structures can be very useful for applications including security documents and publicity. At the existing systems, the color images are usually produced using the ‘bottom-up’ methodology, i.e. specific color pixels comprising sub-wavelength structures are positioned at the corresponding locations of the substrate, in accordance with the color image. The fabrication process usually utilizes lengthy procedures and costly techniques, such as electron beam lithography (EBL), and focused ion beam (FIB), for each given color image. Chuo et al. (Nanotechnol. 24, 055304, 2013) teaches that one master stamp can be fabricated for a given color image and large quantity of the color images can be replicated from the same master stamp. The limitation is that each new color image requires a new master stamp and the fabrication process may be time consuming. In many applications, for example, for producing only a few security identity photos for a person's image, making a new master stamp for each person is obviously not practical.
Color photography techniques based on photographic films can be rapid and cost-effective in producing color images. However, color pigments and color film emulsion can lead to inconsistencies and fading over time. The color images produced from conventional film-based color photography are easy to be unscrupulously duplicated, and not suitable for security applications.
Other difficulties with existing systems, methods and techniques may be appreciated in view of the detailed description of example embodiments herein below.